The present invention relates to a method for forming a piston having a peg which is applied to a brake of a fluid pressure type.
FIG. 1 shows a known disc brake apparatus. In the disc brake apparatus, a brake body 1 is provided with a cylinder 2 and a cup-shaped piston 3 sealably and slidably mounted inside of the cylinder 2. The brake apparatus is arranged so that an open end of the cup-shaped piston 3 extends from the cylinder 2 and abuts against a backing plate 5 of a brake pad 4. The cylinder 2 forms inside thereof a fluid chamber 18 with an end wall of the piston 3. The end wall 8 of the cup-shaped piston 3 is provided with a peg 9 for providing a gap between the end wall 8 of the piston 3 and a bottom 10 of the cylinder 2 while the peg 9 abuts against the bottom 10. Owing to such a structure, a pressurized brake fluid is readily supplied from an inlet 7. When the brake operation is actuated, the pressurized brake fluid is supplied to the fluid chamber 18 through the inlet 7 to thereby actuate the end wall 8 of the piston 3 so that the cup-shaped piston 3 urges the brake pad 4 against a rotor 6 and then the brake is performed.
In the conventional method, the cup-shaped piston having the peg is formed in accordance with steps shown in FIGS. 2A to 2C. That is, a metallic material 11 is cut out to have a predetermined measure as illustrated in FIG. 2A. In the next step shown in FIG. 2B, the metallic material 11 is cold forged to form a cylindrical preform having a flat top surface 12 and a cornered edge portion 20. Finally, the preform is finished to form a cup-shaped piston 3 having a peg 9 on the top surface of the end wall 8 at a final step shown in FIG. 2C. Thus, the cup-shaped piston 3 is serially formed.
As described above, in the conventional method for forming a cup-shaped piston after the metallic material 11 is preformed in the step shown in FIG. 2B, the preform is directly finished to form the cup-shaped piston having the peg. However, such a conventional method shown in FIG. 2A to 2C may cause a difficulty as shown in FIGS. 3A to 3C. In the drawings, the step shown in FIG. 3A corresponds to that shown in FIG. 2B and FIGS. 3B and 3C correspond to FIG. 2C. FIG. 3B is a top view of the cup-shaped piston and FIG. 3C is a sectional view cut out by a line X--X of FIG. 3B. Specifically, in the conventional method since the peg of the cup-shaped piston is projected directly from a flat top surface of the end wall of the piston, the peg may have a relatively large drop part D1 as shown in FIGS. 3B and 3C so that it is difficult to obtain a precise measure.
Further, since a preferred material flow at a part D2 of the cornered edge portion may be deformed due to the projecting portion 9 as illustrated in FIGS. 3B and 3C, it would be difficult to obtain a required precise measure of the piston. Furthermore, the conventional method raises a possibility to cause the cornered edge portion to be chipped off or cracked. The piston having such a difficulty would have a problem that if the piston is applied to a brake apparatus of a fluid pressure type, the chipped or cracked portion may be rusted due to the aqueous brake fluid since the chipped or cracked portion would not be sufficiently plated.